As is known, the tread of a tire undergoes wear that results from the rolling of the tire. If the tread has grooves, the effect of this wear is to reduce the depth of the grooves, thereby degrading the performance of the tire on wet ground. On reaching a certain wear level, the tire no longer enables rolling to take place under optimum security conditions. The tire or its tread (“retreading”) is then replaced or, if the residual thickness of the tread so allows, the grooves of the tread are cut out afresh.
The wear level of a tread can be monitored by measuring the depth of the grooves, but that operation requires a suitable measurement tool, which makes it rather troublesome. It has therefore long been proposed to position wear indicators on the rolling surfaces so as to alert the driver of the vehicle visually when the minimum tread groove depth has been reached. The use of colored rubber has often been envisaged: for example, patent FR 1 480 472 describes a tread containing colored materials that contrast against those of the tire, which appear when the wear limit permitted for the treads has been reached. Patent FR 797 713 proposes that a rubber plug should be incorporated in the tread, the said plug being divided diagonally into two differently colored parts. As the tread becomes worn, the distribution of the colors visible at the surface changes; when the visible part of the plug has changed color completely, the tire is due for changing. U.S. Pat. No. 4,226,274 describes a more recent example of implementing the same principle. However, the use of colored rubber has a number of drawbacks, particularly relating to the complexity and cost of the fabrication process; exact positioning within the tread is difficult to ensure. Moreover, inserts of colored rubber may give rise to irregular wear.
The use of colored rubbers can be avoided by taking advantage of the visual contrast between the rubber forming the tread and a groove or cavity in the said tread. Patent GB 1,279,966 describes a tread provided with grooves of different depths distributed across the width of the tread: as the tire becomes progressively more worn, the number of grooves decreases; the number of grooves that remain allows conclusions to be drawn about the wear level of the tread. The same principle is also adopted in patent application EP 0 250 113 and patent EP 0 853 553; in the latter, cavities of different depths are distributed all around the circumference of the tire. This approach is noted for its ease of implementation; but it has one major disadvantage: the number of grooves or cavities present on the rolling surface at a given time during the tire's life does not give a precise indication of the wear level. In other words, although the user can perceive the state of wear progress visually, he does not know where the tread is in its “life cycle” unless he has information about the tread's geometry in its initial and final conditions.
Two approaches have been proposed to overcome that difficulty:
Patent application US 2002/0036039 proposes a wear indicator comprising two cavities of the same depth but with different geometries. The cross-section of one cavity in the rolling surface becomes smaller as the tire wear increases, while the cross-section of the other cavity remains constant. A comparison of the cross-sections of the two cavities indicates the wear level of the tread. This approach has the advantage of allowing a directly “readable” measurement of the tread's wear level; its drawback is that is requires two cavities of different shape or orientation, which increases the incision fraction of the rolling surface and can result in the creation of zones with uneven wear.
U.S. Pat. No. 3,653,422 adopts a different approach. A plurality of cavities with different depths are combined to form a wear indicator; each cavity has a cross-section whose shape contributes towards forming a message for the attention of the tire's user; as the tire becomes progressively more worn, the message changes. U.S. Pat. No. 6,523,586 uses the same idea by associating several cavities each of which has a trace (in the rolling surface) that corresponds to a number which expresses the residual thickness (in millimeters) the tread will have at the time when wear obliterates the cavity in question. Thus, when a tread of initial thickness 10 mm is reduced by wear to a thickness of 6 mm, the wear indicator, initially composed of nine cavities forming the numbers 1 to 9, will still have five cavities whose sections in the rolling surface correspond to the numbers 1 to 5. This system has the advantage of displaying the residual tread thickness as an absolute value, but it has some disadvantages. Firstly, it requires a large number of cavities, particularly if it is desired to determine the wear level at several positions across the width and periphery of the tread. Secondly, the user can be confused by the facts that several values are displayed at the same time, and that he must remember only to take note of the highest value (in the example above, when the residual thickness is 5 mm the numbers 1 to 5 appear on the rolling surface). Finally, it is related to a particular measurement system (the metric system in the example given) and requires adaptation in regions or for users unfamiliar with that system.
More recently, another type of indicator has been proposed which allows to detect the moment in time when the tires of a vehicle have to be interchanged (“rotated”). Patent application EP 1 574 363 describes an indicator formed by a cavity whose trace on the rolling surface changes as a function of the tread's wear level. When a given threshold is reached, the shape of the trace is radically changed, for example from a circular shape to a squared shape or vice versa. Such an indicator has the advantage of visibly signaling that a certain wear threshold has been reached, but it is difficult to identify the wear level at the moment of observation. The user has to look up a reference in order to know what the actual wear level is. This is particularly true when the indicator allows to distinguish several wear levels (as does the indicator of FIG. 3(h) of the cited patent application).